The invention relates to a turbine for an exhaust gas turbocharger with a turbine wheel disposed in a turbine housing and a guide vane structure arranged in the turbine inlet flow passage so as to be variably adjustable.
DE 10 2008 053 169 A1 discloses a turbine for an exhaust gas turbo-charger with variable turbine geometry, which comprises guide vanes. The guide vanes are rotatably supported between a guide vane carrier and a shroud cap. The guide vanes serve to guide the exhaust gas of a combustion engine so as to provide for an aerodynamically efficient flow of the exhaust gas to a turbine wheel of the turbine.
Because the exhaust gas flows around the guide vanes, the guide vanes are subjected to very high exhaust gas temperatures. In order to avoid jamming of the guide vanes, so-called functional gaps are to be provided between the guide vanes and the guide vane carrier as well as between the guide vanes and the shroud cap. These functional gaps allow the guide vanes to expand when they are subjected to high temperatures so that no jamming occurs. These functional gaps are required both for Diesel engines and for Otto engines. In Otto engines, particularly high exhaust gas temperatures occur, in particular compared to Diesel engines, so that the functional gaps in Otto engines have to be designed very large.
As an alternative to rotatable guide vanes, the use of stationary guide vanes relative to the turbine casing of the turbine is known, which, by means of a shroud element, may be covered at least partially in a first position whereas in at least a second position of the shroud element the vanes may be exposed to the hot exhaust gas flow. The shroud element may be displaced in the axial direction of the turbine relative to the turbine casing. Such a displaceable element may to some extent eliminate the above described functional gaps which may lead to undesired flow losses. The displaceable element which partially covers the guide vanes in the radial direction is, however, of a relatively high complexity, in particular in combination with adjustable vanes, which results in high costs of the turbine.
It is therefore the object of the present invention to provide a turbine or an exhaust gas turbocharger which may efficiently be adapted to varying exhaust gas mass flows and which does not involve high costs.